No Fertilization Is Optimal, but a Low Level of Fertilization Is an Acceptable Compromise for Conserving Lowland Hay Meadows Under Voluntary Agri-Environmental Schemes in Luxembourg

Temperate grasslands rank among the most diverse ecosystems in the world at small spatial scales. Maintaining species-rich grasslands is therefore essential for biodiversity conservation in Europe. Several EU countries have implemented Agri-Environmental Schemes (AES) designed to preserve grassland biodiversity and support farmers in developing their operations in a sustainable manner. While the prohibition of fertilization helps maintain oligotrophic, species-rich grasslands, it remains a stringent requirement that may discourage farmers from enrolling in these schemes. Because hay meadows are mesotrophic habitats, low levels of fertilization could potentially allow the maintenance of satisfactory biodiversity. We compared two AES measures implemented in Luxembourg: P4B (mowing after 15 June, no fertilization) and P3B (mowing after 15 June, fertilization permitted up to 50 kg N ha−1 year−1), the latter aiming to increase forage yield while maintaining acceptable biodiversity levels. Nineteen grassland sites were selected across Luxembourg. Within each site, adjacent delimited experimental plots were managed under P3B (350 kg ha−1 of mineral fertilizer, N14–P9–K24) and P4B (no fertilization). Floristic composition and biomass yield were recorded annually over four years. Fertilization had a negative effect on hay meadow biodiversity and conservation status, resulting in fewer typical species, a reduced dicot-to-grass ratio, and greater cover of nitrophilous species. Nevertheless, most fertilized plots still met the criteria for hay meadows of good to medium conservation status according to Luxembourg’s evaluation framework. Fertilization increased biomass yield by approximately 40% compared with unfertilized plots. We conclude that P4B remains the optimal management option for hay meadows and should be prioritized in areas hosting sensitive species and in the most biodiverse sites. However, P3B represents a pragmatic compromise, maintaining an acceptable level of biodiversity while being more attractive to farmers.